1. Field of the Invention
The present invention relates to a light-emitting device used in the field of lighting and an illumination apparatus using the same.
This application claims priority from Japanese Patent Application No. 2005-079059, filed Mar. 18, 2005 in the Japanese Patent Office, the entire content of which is incorporated herein by reference.
2. Description of the Related Art
In the field of lighting, there is great expectation for solid state lighting, particularly in regard to white lighting using semiconductor light-emitting diodes, which are the target of extensive and aggressive research and development efforts. White light-emitting diode lamps have achieved light-emitting efficiency equal to or higher than incandescent lamps, and have the potential of further improvement. It is anticipated that they will come to be used extensively as energy-saving illumination apparatuses in the near future. It is advantageous that white light-emitting diode lamps do not contain substances that impose an environmental load, such as mercury. Because of their small size, white light-emitting diode lamps are incorporated in backlights of liquid crystal display devices and mobile phones and are widely used.
The present invention relates to a light-emitting device used in the field of lighting and provides a light-emitting device having high efficiency, excellent long-term reliability, and various emission colors, such as, for example, daylight, neutral white, white, warm white, and incandescent lamp colors, for improved color rendering, and an illumination apparatus using the same.
In the prior art, white light-emitting diode lamps (hereafter referred to as LEDs) are known which use an LED element that emits light of short wavelengths such as blue light and a fluorescent (phosphor) material that absorbs and is excited by all or a part of the light emitted by the LED element and emits fluorescence of longer wavelengths such as yellow fluorescence. For example, a white LED lamp including a compound semiconductor blue LED element and a cerium-activated yttrium-aluminum-garnet (hereafter referred to as YAG) fluorescent material that absorbs blue light and emits yellow, a complementary color to blue, fluorescence is described in Japanese Patent Publication Nos. 2900928 and 2927279 and in K. Bando, K. Sakano, Y. Noguchi, and Y. Shimizu, “Development of High-bright and Pure-white LED Lamps,” J. Light & Vis. Env.; Vol. 22, No. 1 (1998), pp. 2-5.
Adding a red light-emitting material to compensate for the red constituent is a known technique. Japanese Unexamined Patent Application, First Publication No. 2003-273409 discloses a technique in which a red fluorescent material such as (Sr1−x−y−zBaxCay)2Si5N8:Euz2+ or SrS:Eu, (CaxSr1−x) S:Eu2+ is added to a white LED including a blue LED element and a cerium-activated YAG fluorescent material. Similar techniques are disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-321675 and in M. Yamada, T. Naitou, K. Izuno, H. Tamaki, Y. Murazaki, M. Kameshima, and T. Mukai, “Red-Enhanced White-Light-Emitting Diode Using a New Red Phosphor,” Jpn. J. Appl. Phys. Vol. 42, (2003) pp. L20-L23.
Further, a technique to realize a white LED using a blue LED element, a fluorescent material excited by blue light and emitting green light, and a fluorescent material excited by blue light and emitting red light is known and disclosed, for example, in Japanese Unexamined Patent Application, First Publication No. H10-163535. Further, a white LED using SrGa2S4:Eu2+, a fluorescent material excited by blue light and emitting green light, and SrS:Eu2+, a fluorescent material excited by blue light and emitting red light, is disclosed, for example, in Paul S. Martin, “Performance, Thermal, Cost & Reliability challenges for Solid State Lightning,” OIDA Conference, May 30th, 2002.
Recently, there has been extensive research on oxynitride and nitride fluorescent materials. For example, europium (Eu)-activated calcium (Ca) doped α-SiAlON fluorescent material is known from Japanese Unexamined Patent Application, First Publication No. 2002-363554. This fluorescent material is excited by blue light and emits yellow light and, therefore, may be advantageously used as a white LED wavelength conversion material. This fluorescent material is described in detail, for example, by R. J. Xie, N. Hirosaki, K. Sakuma, Y. Tamamoto, M. Mitomo, “Eu2+-doped Ca-α SiAlON: A yellow phosphor for white light-emitting diodes,” Appl. Phys. Lett., Vol. 84, pp. 5404-5406 (2004). A low color temperature white LED lamp using the fluorescent material and having excellent color stability in the presence of temperature changes is described by K. Sakuma, K. Omichi, N. Kimura, M. Ohashi, D. Tanaka, N. Hirosaki, Y. Yamamoto, R.-J. Xie, T. Suehiro, “Warm-white light-emitting diode with yellowish orange SiAlON ceramic phosphor,” Opt. Lett. Vol. 29, pp. 2001-2003 (2004).
Conventionally, fluorescent materials are predominantly oxides or sulfides and must be further improved in terms of durability and their high-temperature properties. Recent efforts have been focused on developing oxynitride and nitride fluorescent materials having improved long-term reliability and high-temperature properties.
Extensive research is taking place regarding solid state lighting devices using a combination of a semiconductor light-emitting element and a fluorescent material, where oxide or sulfide fluorescent materials are the primary materials.
The development of a light-emitting device using an oxynitride or nitride fluorescent material and having high emission efficiency and excellent long-term reliability and high-temperature properties is needed.